This word is often used negatively for when someone is acting too much like a teacher.
“for when …” 是正确的啊。最近读的一篇序言里就有两个地方用到了此结构：
The dictionary contains hundreds of illustrations and diagrams for when processes such as the nitrogen cycle or the digestion of food can be better understood in visual form.
And for when you’re working at your computer, there is the QUICKfind facility, a mini pop-up version of the dictionary that gives you instant definitions of words while you are studying or surfing the Web.
这里 when 意思是：the time or occasion at or in which // tomorrow is when we must decide